Recently, the demand for environmentally-friendly alternative energy sources has become an indispensable factor for future life as the price of energy sources increases due to the depletion of fossil fuels and the concerns on environmental pollution are amplified. Therefore, a lot of research has been focused on various electric power production technologies such as atomic power, solar power, wind power, tidal power, etc., and electric power storage devices for more efficient use of the energy produced as such have been drawing much attention.
Particularly, as technology development and demand for mobile devices increase, the demand for batteries as an energy source is rapidly increasing, and recently, the use of rechargeable batteries as electric vehicles (EVs), hybrid electric vehicles (HEVs), etc., as power sources has been realized, and its application area is expanding to be used as an auxiliary power source, etc., through gridation, and accordingly, a lot of research on batteries that can meet various demands has been conducted.
Typically, there is a high demand for a prismatic secondary battery and a pouch-type secondary battery which can be applied to products such as mobile phones with a thin thickness, etc., with respect to the shape of the batteries, whereas there is a high demand for lithium secondary batteries such as lithium ion batteries and lithium ion polymer batteries, which have advantages such as high energy density, discharge voltage, output stability, etc., with respect to materials of the batteries. Additionally, the secondary battery is also classified according to the structure of the electrode assembly which includes an anode, a cathode, and a separator interposed between the anode and the cathode.
Representative examples may include a jelly-roll type (wound type) electrode assembly having a structure in which long sheet-like anodes and cathodes are wound in a state that separators are interposed therebetween; a stacking type electrode assembly in which a plurality of positive electrodes and negative electrodes cut in units of a predetermined size are sequentially stacked in a state that separators are interposed therebetween, etc. Recently, in order to solve the problems of the jelly-roll type electrode assembly and the stacked electrode assembly, a stacking/folding type electrode assembly, which is an electrode assembly having an advanced structure of a mixed type of the jelly-roll type and the stacking type, in which unit cells where positive electrodes and negative electrodes of a predetermined unit are stacked in a state that separators are interposed therebetween, are sequentially wound in a state being disposed on a separator film, is developed.
Additionally, the secondary battery, according to the shape of the battery case, is classified into a cylindrical battery, a prismatic battery in which an electrode assembly is built in a cylindrical or rectangular metal, and a pouch-type battery in which an electrode assembly is built in a pouch-type case of an aluminum laminated sheet.
Generally, a lithium secondary battery performs a formation process during a manufacturing process, and the formation process is a step of activating the battery by performing charging and discharging after assembling the battery, lithium ions discharged from the cathode at the time of charging move into the anode to be inserted thereto, and in particular, a solid electrolyte interface (SEI) film is formed on the surface of the anode. The formation process generally proceeds by repeating the charging and discharging with a constant current or constant voltage in a certain range.
As such, in the case of the cylindrical battery, the gas generated in the formation conversion process of the battery is concentrated in the winding center portion of the electrode assembly having a relatively small space due to the specificity of the shape of the electrode assembly, thereby forming gas trap, and the gas trap serves as a factor that prevents all portions of the electrode assembly from being completely impregnated in the electrolyte solution, and thus there is a problem in that a lithium precipitation region is generated at the center of the electrode assembly having the gas trap formed thereon.
FIG. 1 is a vertical cross-sectional view schematically showing the structure of a conventional cylindrical battery cell.
Referring to FIG. 1, the cylindrical battery cell 100 is manufactured by receiving a wound type electrode assembly 120 into a cylindrical case 130, injecting an electrolyte into the cylindrical case 130, and coupling a top cap 140 having an electrode terminal (e.g., a positive terminal; not shown) formed thereof into the open top end of the case 130.
The electrode assembly 120 has a structure in which an anode 121, a cathode 122, and a separator 123 are sequentially stacked and wound in a round shape, where a cylindrical center pin 150 is inserted into the wound core (the central portion of the jelly-roll) thereof. The center pin 150 is generally made of a metal material to provide a predetermined strength and is formed of a hollow cylindrical structure in which a plate material is bent in a round shape. The center pin 150 serves to fix and support the electrode assembly 120 and acts as a channel for releasing the gas generated by an internal reaction during charging and discharging and at the time of operation.
However, since the hollow portion of the center pin 150 is relatively narrow and fine, the gas generated during the formation process passes through the center portion of the electrode assembly 120 through the hollow portion of the center pin 150, is concentrated due to the reasons such as the bottleneck phenomenon, etc., thereby forming gas trap.
Generally, to solve the above problems, the gas trap is naturally removed for a sufficient time by charging the spare battery cell in a SOC range with the highest rate of gas trap formation (i.e., in a particular SOC range where the gas is produced in the highest amount) and undergoing an aging process under predetermined temperature and time.
However, since the aging process requires too much time, the process of manufacturing a battery cell may be delayed, and despite the aging process, it is possible that the gas trap inside may not be completely removed, thus reducing the reliability of the process.
Accordingly, there is a high need for the development of a technology that can fundamentally solve the problem.